1. Technical Field
This invention relates in general to computers and, more particularly, to a portable computer with a low voltage differential signaling adapter.
2. Description of the Related Art
For many years, the popularity of portable computers has risen as the size and weight of the portable computer has been reduced. Early portable computers were known as xe2x80x9cluggablexe2x80x9d computers, since they could be transported, but were only slightly smaller and lighter than comparable desktop computers. xe2x80x9cLaptopxe2x80x9d computers were smaller and lighter, but generally had reduced features and flexibility because most of the circuitry needed to be designed into the laptop motherboard without the option of expansion boards.
Notebook computers are significantly smaller and lighter than laptop computers. These computers can easily be carried in a briefcase. The widening use of multimedia in notebook computer systems has put increasing demands on notebook video systems. One area in which notebook computers are less desirable than their desktop countertops is in the video display. While CRT (cathode ray tube) displays commonly can handle resolutions of 1280xc3x971024 and color depths of 16.7 million colors, LCD (liquid crystal display) panels are generally limited to VGA (640xc3x97480) and SVGA (800xc3x97600).
New LCD displays can handle higher resolutions. The enhanced resolution of the new displays, however, requires a higher data bandwidth to pass data from the graphics controller to the display. The need for a higher data bandwidth presents significant problems for the system designer. Increasing the number of lines between the graphics controller, which is housed in the main housing, and the display, which is typically housed in a separate housing coupled to the main housing through hinges, would enlarge the cable which passes through the hinges. In many cases, increasing the cable size presents serious problems to the system designer. Increasing the data rate between the graphics controller and the display would result in an increase in EMI (electromagnetic interference) emissions. Since these emissions are among the most difficult to control, this approach is also not feasible.
Higher data rates could be achieved using low voltage differential signaling (LVDS), which would significantly lower the EMI, thereby allowing higher data rates without exceeding FCC (Federal Communications Commission) limits on EMI emissions. LVDS uses a differential pair to transmit each signal. In contrast, CMOS(complementary metal on silicon)/ TTL (transistor to transistor logic) logic uses a single signal line to transmit each signal; the line""s voltage is measured relative to a common ground to determine whether the signal is a logical high or a logical low. Typical thresholds for a CMOS/TTL logical high signal are voltages at or above approximately 2.0 volts and thresholds for a CMOS/TTL logical low signal are at or below approximately 0.8 volts. By contrast, LVDS signaling uses xc2x1345 millivolt swings to represent logical high and low voltages. Since current notebook designs are based on CMOS/TTL logic levels, and since current graphics controllers do not support LVDS outputs, use of the enhanced resolution panels requires a significant redesign of the system circuitry.
Therefore, a need has risen for a computer which uses LVDS signaling without significant redesign.
The present invention provides a computer comprising a processor coupled to a graphics controller for outputting video information defined using a first signal type. A translation board is removably coupled to the graphics controller for receiving video information defined using the first signal type and outputting video information defined using a second signal type. A display is coupled to the translation board for receiving the video information defined using the second signal type.
The present invention provides significant advantages over the prior art. First, higher resolution displays can be used with an existing system board design, since the translation board can drive an enhanced display while connected to a graphics controller used to drive normal VGA and SVGA displays. The translation board can convert TTL signals from the graphics controller to LVDS signaling, which allows a faster transmission of signals to the display over fewer lines and with less EMI emissions.
Second, the translation board allows the same system board design to be used for both regular VGA and SVGA display panels and for higher resolution panels. In a design which does not require the enhanced outputs, the display can be connected directly to the graphics controller without signal translation. Thus, purchasers who do not need an enhanced display, which comprises a large portion of the overall cost of a notebook computer, can purchase a computer configured with a less expensive display, without purchasing the translation board. For systems using an enhanced resolution display, the translation board can be mounted on the board carrier and connected between the graphics controller and the display. Consequently, a single design can support multiple display types without reserving space on the system boards for LVDS circuitry and without providing multiple connectors for different display types.